New to Web Service

Trying to create a web service for auto complete extender. Saw some tutorials but can't get it to work

<ajx:AutoCompleteExtender ID="AC1" runat="server" ServiceMethod="FindItemId" MinimumPrefixLength="2" CompletionInterval="100" EnableCaching="false" CompletionSetCount="10" TargetControlID="TextBox1" FirstRowSelected="false" ServicePath="~/FindItemId.asmx"></ajx:AutoCompleteExtender>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

/// <summary>
/// Summary description for FindItemId
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
 [System.Web.Script.Services.ScriptService]
public class FindItemId : System.Web.Services.WebService
{
	public FindItemId()
	{

	}

	[WebMethod]
    public static List<string> GetItemId(string prefixText, int count)
	{
		using (SqlConnection conn = new SqlConnection())
		{
			conn.ConnectionString = ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString;
			using (SqlCommand cmd = new SqlCommand())
			{
				cmd.CommandText = "SELECT ItemId FROM CoItems where ItemId like @SearchText + '%'";
				cmd.Parameters.AddWithValue("@SearchText", prefixText);
				cmd.Connection = conn;
				conn.Open();
				List<string> Items = new List<string>();
				using (SqlDataReader sdr = cmd.ExecuteReader())
				{
					while (sdr.Read())
					{
						Items.Add(sdr["ItemId"].ToString());
					}
				}
				conn.Close();
				return Items;
			}
		}
	}
}

<Services><asp:ServiceReference Path="~/FindItemId.asmx" /></Services>

How is it supposed to work?